Automotive vehicle steering columns are well-known which include an elastic coupling unit such at that shown in FIG. 1. This type of steering column connects a steering box to the steering wheel. The steering box comprises a rack 1 and a pinion 2 on which the first element of a transmission assembly is mounted The other end of the steering column is terminated by a cardan joint 8. A second element of this transmission assembly comprises an intermediate axle 9 arranged between the cardan joint 8 and a cardan joint 10. In order to absorb the noises and the vibrations from the steering back-up pump, an elastic coupling unit 6 is mounted on the first element of the transmission assembly, i.e., between the pinion 2 and the cardan joint 8. This elastic coupling unit 6' possesses an input plate 4' and an output plate 5'. The input plate 4' is mounted on a flange 3 which accommodates an extremity of the steering box pinion. The output plate 5 possesses a flange 7' which engages a jaw of the cardan joint 8. In this type of structure, the technology of this transmission element is such that the conical rotation center of the elastic element 6' is located at a point A, approximately on the axis of this transmission element. According to the constitution of the elastic portion 6', this point A may move along the axis and take on a value X, which equals 0 when A located in the middle of the elastic coupling unit, whereas the same value may increase infinitely. Under the effect of the torque transmitted by the driver at the steering wheel, the elastic portion 6' is subjected to an additional torque C around the point A. This additional torque generates a conical defection of the system. This conical deflection causes the B--B' displacement of the cardan joint centre 8, i.e., a rotation centred around the point A with radius AB. This displacement caused by the rotation produces large space requirements for the rotation of the cardan joint 8, which is not compatible with the room available in modern vehicles. Moreover, even if the steering column is arranged so that both cardan joints 8 and 10 have a structure with angles enabling more or less homocentric transmission, the movement along the arc BB' of the center of the carden point at least partially destroys this homocentric assembly.
The supplementary torque which appears depends on the angle described by the cardan joints, i.e., the geometry of the transmission as well as the input torque applied by the driver; and this supplementary torque may reach values in the order of half the torque to be transmitted. The displacement of the cardan joint 8, i.e., of the centre position of its point B to the point B' also depends on the ratio of the lever arms and may reach values relatively high which are inadmissible in modern vehicles. The position of the point A is random and one never knows exactly where it is. Indeed, in case when the disk in elastic material of the elastic coupling unit 6 is very flexible under shearing effect and very stiff under compression, the point A is located towards the infinite, since there can only be displacements in the shearing direction, i.e., in the direction perpendicular to the axis, hence without any rotation movement. On the contrary, when the elastic disk of the elastic coupling unit 6' is very stiff under shearing effect and very flexible under compression, the point A is located in the center of this elastic disk. As we never know exactly the structure nor the precise characteristics of this disk in elastic material, we do not master the position of the point A, and the displacement of the point B is always relatively important. In order to remedy these drawbacks, the well-known solutions tend to increase the conical stiffness and hence the torsional stiffness, which causes the major drawback to .decrease the noise and vibrations absorption capacities for which this coupling unit had been designed initially. Finally, the supplementary torque varies twice per rotation with two points located at maximal value, and two points located at minimal value, both in the same direction. This supplementary torque and its evolution during rotation, destroy the rubber of this elastic coupling unit quite rapidly.